Slag conditioner



United States Patent 3,396,010 SLAG CONDITIONER Herbert E. Gould, Seattle, Wash., assignor to Northwest Olivine Company, Seattle, Wash., a corporation of Washington No Drawing. Filed Sept. 16, 1965, Ser. No. 487,899 17 Claims. (Cl. 75-30) ABSTRACT OF THE DISCLOSURE Olivine is added in the basic production of iron and steel to increase slag fluidity. The addition of olivine may be made along with the charge and both in oxidizing and refining slags.

This invention relates to basic production of iron and steel. More particularly, this invention relates to increasing the fluidity of oxidizing and refining slags in basic processes for producing iron and steel by use of a slag conditioner.

One of the primary aims of slag control is to produce a slag that will eliminate phosphorus and sulfur to the desired extent without excessive use of lime (which increases slag viscosity) or excessive loss of iron as oxide. Since slag control is aimed at governing the rate of oxidation of the molten bath, it is therefore concerned primarily with regulating the content of iron oxides and the consistency or viscosity of the slag. About the only control that a furnace operator has over the iron oxide content is through regulation of the slag viscosity, which exerts a major influence on the rate of oxidation of the bath by the slag. In the same way in which the basicity and therefore the viscosity of the slag control the FeO content in the slag, it is possible to control the phosphorus and sulfur content of the bath.

It is a common practice to add fluorspar to the slag to increase its fluidity (lower its viscosity) to aid in the removal of phoshorus and sulfur to establish a desired basicity with a desired FeO content. Fluorspar makes it possible for a slag to have a high lime content without being excessively viscous. Until the present time, addition of fluorspar has been the primary means by which a furnace operator could exercise maximum and consistent control over slag conditions. Addition of silica sand has been attempted and was found unsatisfactory because it lowers the basicity of the slag. Addition of fluorspar, however, is now becoming less desirable because good metallurgical grades of fluorspar are becoming scarce and other grades of fluorspar contain sulfur and antimony as major contaminants, both of which are highly undesirable contaminants in basic steel making processes, which are not removable from fluorspar.

In addition to this scarcity of good metallurgical grades of fluorspar, use of fluorspar has always been a problem because of its extremely corrosive action to the basic linings of furnaces. Furthermore, use of fluorspar results in the formation of hot fluorine gas which if permitted to do so, will condense into hydrofluoric acid. Consequently elaborate and expensive entrapment facilities must be provided to prevent the formation of hydrofluoric acid.

A primary object of this invention is to provide a means for controlling the fluidity of oxidizing and refining slags in basic processes for producing iron and steel without creating corrosion problems of the type caused by fluorspar. Another object of the invention is to provide for controlling refining slag viscosity Without introducing contaminants into the bath and without reducing the basicity of the slag. A further object is to provide for enriching a cupola charge in a basic cupola furnace. Still another object is to aid in the removal of phosphorus and sulfur by increasing fluidity. These and other objects and advan- "ice tages of this invention will be apparent from the following description.

Oxidizing and refining slags produced in accordance with this invention are applicable to all basic steel processes such as basic open hearth, basic electric, basic induction and basic oxygen steel making processes such as the L-D process. Slags produced in accordance with this invention also find utility in water-cooled basic cupola furnaces for producing iron.

In general basic oxidizing and refining slag conditioners are produced in accordance with this invention by adding olivine to the slag in the presence of calcium oxide. Calcium oxide may be provided in the form of calcined lime or limestone. Preferably olivine is added in an amount between about 2 to 28 lbs. per ton of steel or iron in the presence of calcium oxide with the olivine sand being about 235% by weight of the calcium oxide. Within this range, basic induction processes would require the least amount, the basic L-D or oxygen process would require the most, and the basic open hearth and basic electric would require the intermediate amounts. If particularly dirty scrap charge is used, more olivine would be used, because more fluxing would be required and a proportionately greater amount of calcium oxide would be used.

Olivine preferably has an FeO content less than about 10% and a 2MgO.SiO content of at least A preferred olivine bearing mineral comprises 92% olivine, 5% enstatite, 1.5% serpentines, and 1% chromite (all percent by weight). A typical chemical analysis of this olivine bearing mineral is as follows:

TABLE I Percent MgO 42.0-49.0 SiO 41.2 F6203 A1203, MHO, Cr O 1.8 CaO 0.2 Ignition loss 0.3-1.5

Particle sizes as small as 70 mesh (U.S. standard screen scale) and as large as 1 inch diameter have been used. In general, a particle size between about 1% inch diameter would be chosen. For very large furnaces such as would be used in L-D processes, particle sizes even larger than 1 inch diameter would probably be used.

It has been found that olivine works much quicker than fluorspar and just as thoroughly in reducing slag viscosity. It has also been found that olivine very definitely aids in the removal of phosphorus and sulfur from the melt. The MgO content of olivine is particularly useful in absorbing phosphorus and sulfur.

The use of olivine sand in the presence of calcium oxide is desirable in basic cupola practice for producing iron. Basic cupola furnaces are water-cooled and usually are not provided with any lining or at least not provided with a corrosion-resistant lining. It has been found that the addition of olivine sand to enrich a basic cupola operation provides suflicient MgO to achieve high absorption of sulfur and phosphorous without adding a corro sive agent to the charge. Olivine may be added in an amount between about 2 to 28 lbs. per ton of steel in the presence of calcium oxide 'with the olivine sand being about 2-35% by weight of calcium oxide. Olivine sand having a composition as specified in Table I and in the paragraph accompanying Table I is suitable.

Generally, the charge of the basic open hearth furnace consists of pig iron, purchased scrap, foundry returns, (gates, risers, and the like) lime and ore. The lime could be high in CaO and burned lime is usually used to maintain a uniform CaO content. Iron ore or oxygen additions are made to the bath as soon as it has melted and reached a temperature in excess of about 2700 F. in

sufficient quantities to produce a long vigorous boiling action during the oxidation period. The refining period which is actually the last part of the oxidation period, requires the further addition of lime in order to increase the CaO content of the slag, and according to this invention, the addition of olivine to keep the slag in a fluid condition. When the carbon content has dropped to the desired point, further drop in carbon content is stopped by the addition of dexodizing alloys, such as FeSi, which combine with the oxygen, removing it from reaction with carbon. The heat is then tapped.

Any discussion of slag control must take into consideration the fundamental working of the basic open hearth process. The charge contains elements such as carbon, silicon, phosphorus, and sulfur, which must be removed by oxidation to produce steel. The chief source of the oxygen required is the flame which is strongly oxidizing toward the slag and steel. Oxygen transfer to the bath is dependent upon the concentration in the slag of the oxygen carriers. Since the slag control is directed toward governing the rate of oxidation of the bath, it is therefore primarily concerned with regulating the content of iron oxides and the consistency or viscosity of the slag. As the basicity of the slag increases during melting, the total iron oxides in the slag is determined by the carbon content of the bath where the slag is sufiiciently fluid. The viscosity of the slag, therefore, exerts the major influence on the rate of oxidation of the bath by the slag. The addition of olivine in accordance with the invention has been found to be particularly useful in controlling slag viscosity.

A typical example of the use of olivine as a slag conditioner in a basic open hearth heat would be as follows:

Example I Sizes of heat: 50,000 lbs.

Composition desired: Carbon, 0.25 to 0.30%; manganese, 0.65 to 0.75%; silicon, 0.35 to 0.45%; phosphorous and sulfur, 0.035% max.

Pounds Percent Charge:

Pig Iron 7, 500 15 Forgings 9, 000 18 Heavy Melting 17, 000 35 Punchings 8, 000 16 Cast Steel 8, 000 16 50,000 Lime 2, 500

Time after start, hrs.: Activity 0 Started charging. 2 hr Finished charging. 3 hr Melted=iron ore addition 500 lbs. (1.0%). 3 hr Lime addition 750 lbs. 3% hr Olivine addition 75 lbs. 4 hr FeSi addition 400 lbs. 4% hr Tapped.

The basic electric process is divided into an oxidizing period and a reducing or refining period. Both periods are important but it is the reactions that take place during the reducing periods that make the process distinctive. The slag-making materials are primarily lime with a high CaO content, carbonaceous material and, in accordance with this invention, olivine. As in the basic open hearth process, the primary function of the olivine is to lower viscosity of the basic slag and to indirectly aid in the removal of sulfur from the steel by the decrease in slag viscosity. Olivine makes it possible for a slag to have a relatively high CaO content and yet still be a fluid slag. The charge may consist essentially of foundry scrap and purchased scrap. The charge is melted and a small amount of lime is added to protect the metal from over-oxidizing. Since there are no oxidizing gases present as in the open hearth process, the oxidation may be controlled by the amount of oxygen in the form of iron oxide that is admitted to the furnace-The solid oxide may be in the form of iron ore or rusty scrap. When the oxidizing period is at an end, one of two refining slag procedures may be used; one being a double slag procedure wherein the oxidizing slag is poured off and a reducing slag added, and the other being a single slag procedure wherein the oxidizing slag is converted to a reducing slag by proper additions.

A typical example of the use of olivine as a slag conditioner in a basic electric double-slag heat would be as follows:

Analysis desired: C, 0.25%; Mn, 0.65%; SiO, 0.30%; P, less than 0.03%; S, less than 0.03%.

Charge: Lbs. Misc. scrap 13,000 Foundry returns 5,000

Time after start, hrs.: Activity 0 Power on.

/2 Lime addition 150 lbs.

1 Lime addition 200 lbs.

1 /2 Lime addition 150 lbs.

2 Melt down.

2 Slag off.

2% Add: ferromanganese, 105 lbs.,

80% Mn; ferrosilicon, lbs., Si; pulverized carbon, 20 lbs.

Add refining slag mix: 200 lbs.

lime, 45 lbs. olivine.

3 Add refining slag mix: 200 lbs.

lime, 25 lbs. olivine, 10 lbs. pulverized carbon.

3% Add pulverized carbon 6 vlbs., and pulverized FeSi 11 lbs. to slag.

3 /2 Add pulverized carbon 4 lbs. to

slag.

4 Add ferromanganese lbs, Mn; ferrosilicon lbs., 50% Si.

4 /2 Tapped.

A typical example of the use of olivine as a slag condition in a basic electric single-slag heat would be as follows:

TABLE III Analysis desired: C, 0.25%; Mn, 0.60%; Si, 0.30%; P and S, less than 0.04%.

Time after start, hrs.: Activity 3 Add: Pulverized carbon 5 lbs.

Add: Wash metal 90 lbs,

3.50% C. Add: Ferromaganese 11 lbs., 80% Mn; ferrosilicon 60 lbs., 50% Si.

3% Tapped.

The use of olivine as a slag conditioner would also be suitable in basic electric processes such as the aluminaslag modification of the double-slag process in which aluminum is used in place of carbon to obtain a reducing slag; and in alloy-steel production in which nickel, copper, or molybdenum are charged with the scrap wherein the usual refining slag is placed on the bath.

What is claimed is:

1. A slag conditioner for use in basic iron and steel processes consisting essentially of olivine sand and calcium oxide.

2. A slag conditioner according to claim 1 wherein the olivine has a FeO content less than about 10% and a ZMgSiO content of at least about 80%.

3. A slag conditioner according to claim 1 wherein the olivine is a principal component of an olivine bearing mineral having a chemical analysis of about the following:

Percent MgO 42.049.0 Si 41.2 F6203 A1203, MIIO, CI203 CaO 0.2 Ignition loss 0.3-1

4. In a basic process for producing steel comprising charging a furnace with metal, melting the charge, oxidizing the charge and then refining the melt, the improvement which comprises adding olivine to the charge as a slag conditioner.

5. A process according to claim 4 wherein olivine is provided in an amount between about 2 and 28 lbs. per ton of steel.

6. A process according to claim 4 including adding calcium oxide to the charge; and wherein olivine is added to the charge during refining of the melt in the presence of 2 to 35 wt. percent of the calcium oxide.

7. A process according to claim 4 wherein olivine is added to the charge during refining of the melt in an amount between about 2 to 28 lbs. per ton of steel.

8. A process according to claim 4 including adding calcium oxide to the charge; and wherein olivine is added to the charge during oxidizing the charge in the presence of 2 to 35 wt. percent of the calcium oxide.

9. A process according to claim 4 wherein olivine is added to the charge during oxidizing the charge in an amount between about 2 to 28 lbs. per ton of steel.

10. A process according to claim 4 wherein the olivine has an FeO content less than about 10% and a 2Mg.SiO content of at least 80%.

11. A process according to claim 4 wherein the olivine is a principal component of an olivine bearing mineral having a chemical analysis of about the following:

Percent MgO 42.0-49.0 Si0 41.2 F6203 A1203, M110, CI'203 1. CaO 0.2 Ignition loss 0.31.5

12. A process according to claim 4 wherein the furnace is a basic open hearth furnace.

13. A process according to claim 4 wherein the furnace is a basic electric furnace.

14. A process according to claim 4 wherein the furnace is a basic induction furnace.

15. A process according to claim 4 wherein the furnace is a basic oxygen furnace.

16. In a basic process for producing iron in a cupola furnace wherein the cupola is charged with metal, the improvement comprising adding olivine as a slag conditioner to the charge to fluidize the cupola charge to improve absorption of sulfur and phosphorous by the slag.

17. A process according to claim 16 wherein the olivine is a principal component of an olivine bearing mineral having a chemical analysis of about the following:

Percent MgO 42.0-49.0 Si0 41.2 Fe O 8.0-14.0 A1 0 MnO, Cr O 1.8 CaO 0.2 Ignition loss 0.3-15

References Cited UNITED STATES PATENTS 2,027,868 1/ 1936 Kinzel 54 FOREIGN PATENTS 576,265 5/1959 Canada.

BENJAMIN HENKIN, Primary Examiner. 

